PWM Inverter device

ABSTRACT

An inverter device operating under pulse width modulation (PWM) principle in which chopping pulse is formed by crossing points of a triangle waveform and a function waveform in order that pulse width of the inverter input current decreases towards later half of pulse train in each period of pulse repetition series so as to make the average input current for each one of the pulses substantially constant.

BACKGROUND OF THE INVENTION

The present invention relates to an inverter operating in a pulse widthmodulation (PWM) principle or to a modulation system in the PWM inverterapparatus.

In a conventional PWM inverter, the control is effected by using a socalled square wave shaped voltage obtained by chopping a certain dcvoltage so that the waveform of the chopped current may differ at eachof the PWM period and accordingly the value of the commutating currentvaries greatly depending on the pulse series even under a same loadcondition. More especially, the duration of one cycle of the inverter isprolonged in counter proportion to the output frequency of the inverterin low speed frequency range. In view of this fact, the number oftriangle wave signal forming the carrier signal in this duration or thepulse mode is generally increased at lower speed and thus the peakcurrent at current commutation is suppressed. In this case, the waveformof the input current for the inverter assumes equivalently the sameenvelope with the motor current formed essentially by RL circuit andincreases exponentially so that the current of the pulse voltage at eachtermination period of the repetition pulses increases also exponentiallyeven the aforementioned increase of the pulse mode. Due to this fact thethyrister element and the current commutating circuit elements formingthe inverter device should be selected to withhold the maximum current.The frequency components contained in the inverter input current includehigher harmonic components of 6 times higher than the inverter outputfrequency beside the abovementioned chopping frequency, since the inputcurrent flows from the feeder through a filter in a form of three-phasefull-wave rectification of the motor current in general by the inverter.By this reason, at the low frequency time such as the starting time, thefeeder current may not be sufficiently smoothed by a decrease of thefilter efficiency so that the feeder current may contain ripplecomponents. Moreover in the rail constituting return path, in which theinverter input current flows, there is also flow of security signalcurrent which is in general in the commercial frequency. Accordingly,when an electric train is driven by a PWM inverter, the frequency rangeof the security signal system passes the device in the low frequencyrange especially at the starting time since the filtering effect isdecreased at such low frequency range and certain signal trouble mightbe caused.

SUMMARY OF THE INVENTION

The present invention is to improve the aforementioned disadvantages ofthe prior art devices and is to realize a modulation system being ableto ease the current commutating duty of the inverter and to decrease thecurrent of lower harmonic component so as to obtain a small and lightweight device.

DESCRIPTION OF THE DRAWINGS

The invention will now be described by referring to the accompanyingdrawings, in which:

FIG. 1 is a block diagram of a typical conventional circuit arrangementof an inverter modulating device;

FIG. 2 is waveform diagram for various parts for explaining theoperation of the device shown in FIG. 1;

FIG. 3 is a block diagram of an inverter modulating device according tothe present invention;

FIG. 4 is signal waveform diagram for explaining operation of the deviceof FIG. 3;

FIGS. 5 to 7 are waveform diagrams for explaining effect of themodulating part of the inverter device; and

FIG. 8 is a block diagram of a modified embodiment according to thepresent invention.

DESCRIPTION OF PREFERRED EMBODIMENT

The invention will now be described by referring to the accompanyingdrawings. In order to help clear understanding of the invention at firsta conventional device will be explained by referring to FIGS. 1 and 2.This conventional device is the case of a 3-phase inverter, wherein m=3,m representing the number of phases.

FIG. 1 shows a block diagram of a basic construction of modulation partof an inverter of a conventional type. In this figure, 1 designates anoscillator, 2 a frequency divider, 3 a three-phase signal distributor, 4a triangle wave generator, 5 a modulator, 6 a gate controller, 7 aninverter, and 8 is an ac motor.

Operation of the device shown in FIG. 1 will be explained by referringto waveforms shown in FIG. 2.

A frequency instruction A₁ for designating the operation frequency ofthe inverter is applied to the oscillator 1 and this oscillator 1generates a pulse series having frequency being an integer multiple ofthe operating frequency of the inverter. The frequency divider 2 dividessaid frequency of the pulses up to minimum necessary multiplex frequencyfor deriving respective phase voltage signal of the inverter 7 andapplies it to the triangle wave generator 4. On the other hand thethree-phase signal distributor 3 is also given the output pulse seriesfrom the frequency divider 2 and it converts into respective phasesignal. At the same time the three-phase signal distributor 3 issupplied with the output signal of the modulator 5 as the choppinginstruction A₄ and modulates said respective phase signal by thisinstruction A₄. Based on the output of the three-phase signaldistributor 3, gate signal for switching element in each phase of theinverter 7 formed for instance of a thyristor is delivered from the gatecontroller 6. This gate controller 6 may also generate a signal forcontrolling current commutating operation of the switching elementforming the inverter 7 when the element requires such currentcommutation. The aforementioned chopping instruction A₄ can be obtainedin the following manner.

A triangle wave carrier signal A₃ at the output of the triangle wavegenerator 4 is compared with a level signal A₂ of constant voltage beinga voltage instruction and for instance, if the carrier signal A₃ ishigher than the level signal A₂, the output of the modulator 5 is madeas a logic value "L" and on the contrary when the level signal A₂ ishigher than the carrier signal A₃, the output of the modulator 5 is madeas a logic value "H." By the chopping instruction A₄ obtained in theforegoing manner, each phase voltage signal of the three-phase signaldistributor 3 is modulated. The one phase voltage waveform V₁ beingapplied between motor terminals U and V is for example as shown in FIG.2(c). A voltage being applied between motor terminals V and W delayed by60° electric angle from said voltage waveform V₁ is represented by phasewaveform V₂ which is shown in FIG. 2(d). A current waveform I₁ flowingthrough the ac motor 8 by application of the phase voltage waveform V₂is shown in FIG. 2(e). Since the carrier signal A₃ should be integralmultiple of the inverter operating frequency, the frequency divider 2 isin general formed in the following way. As an example of a most simplestcircuitry being easily conceived by those skilled in the art inelectronics is that to make frequency division in 1/2 at 1st stage, andto make further division to 1/4 at 2nd stage, or in general to 2^(-n) (n. . . number of steps and n=1,2,3. . .) in each successive stage. If weassume the number of steps is 4 (n=4), the output frequency of the finaloutput stage becomes 1/16 of the input pulse frequency. Also we mayobtain either 1/2, 1/4, or 1/8 as an intermediate output. From this andusing these synchronizing pulses the triangle wave generator 4 iscontrolled and the desired triangle wave signal in an integer multipleof the inverter operating frequency can be obtained.

According to the aforementioned conventional modulation system of theinverter, the inverter current during 1/6 cycle of the inverter fromtime t₁ to time t₂ may have a waveform as shown in FIG. 2(e) whichincreases according to the lapse of time. The dc input current flowingin the inverter 7 is equal to the fullwave rectification of therespective phase current and it assumes a waveform of repetition of thehatched portion shown in FIG. 2(e). As can be seen from this example,the difference of current values at beginning end and at terminating endof a period starting from an instant t₁ and terminating at an instant t₂assumes ripple component of the inverter input current including 6 timeshigher harmonics of the inverter output frequency. This will cause anincrease of the commutating current at the terminating period of thechopping. Furthermore, the aforementioned ripple component may resulttrouble in security signal system although substantial part of the sameis removed by the filter.

The present invention is to provide a novel modulation system of theaforementioned chopping instruction signal. The invention is to decreasethe current ripple in the inverter input current and to effectivelyprevent security signal trouble as well as to realize a compact device.

FIG. 3 shows the modulation part of the inverter according to thepresent invention, in which 9 is a trigger pulse generator and 10 is afunction generator. In the figure, the same parts with that in FIG. 1are designated by the same reference numerals. FIG. 4 is waveformdiagrams for explaining operation of FIG. 3.

The trigger pulse generator 9 shown in FIG. 3 generates trigger pulsesA₅ having very narrow pulse width at predetermined instants synchronizedwith the inverter output frequency. The function generator 10 is formedto derive an output signal having time function starting from an initialvalue at when the trigger pulse is given. In general, this generator 10is of an exponential type being formed of condenser C and a resistor Rusing discharge characteristics of the CR elements. In the aboveconstruction, the condenser C may sufficiently be charged by the triggerpulse having very narrow pulse width by arranging the output impedanceof the trigger pulse generator 9 to be small. After charging of thiscondenser C, the output of the trigger pulse generator 9 becomes zerovoltage. All of the charge storaged in the condenser C by the diode D isdischarged through resistor R and the variation of terminal voltage ofthe condenser C assumes a waveform of exponential function. Thisresistor R may be a fixed value type if the amplitude of the functionwaveform to be added with the level signal A₂, which is a constantvoltage level, can previously be selected according to thecharacteristics of the motor. This fixed value should be one forobtaining the desired voltage division ratio and the desired resistancevalue for discharge. The output of the function generator 10 produced inthe aforementioned manner is added with the level signal A₂, which issame as shown in FIG. 1, in the adder 11 and an exponential functionwaveform A₆ as shown in FIG. 4(b) is obtained.

Accordingly, an inverter input current assumes waveform as shown by thehatched line portion of FIG. 4(e). The current waveform I₁ varies fromone having a small initial value and wide pulse width to one havinglarger initial value and narrower pulse width. The total area of thehatched portion is nearly the same for each of the pulses andaccordingly the average value of the inverter input current havingnearly the same value can be obtained. The above explained embodiment ofthe present invention can be operated in the same manner with theconventional device as shown in FIG. 1 by applying an interruptioninstruction A₇ to the control terminal of the trigger pulse generator 9and hence by stopping the generation of pulses. Therefore, when thefiltering effect is sufficient by an increase of the inverter frequencyor when the effect of the present invention is not required by somedecrease of number of pulses of the chopping instruction A₄, thefunction of the present invention can be stopped. It is very easy toalter the output waveform of the function generator 10 in a sawtoothform or other function waveform by using the generating functionthereof. Also it is easy to alter the output signal to match thevariation of the inverter output frequency to assume always a similarwaveform.

The advantage of the modulation system according to the presentinvention will be explained in more detail by referring to FIGS. 5 to 7.

FIG. 5 shows inverter input current waveform in the conventional systemas shown in FIG. 1, in which equip-interval pulse width modulation iseffected by the modulation by the level signal A₂ and the carrier signalA₃. In this prior art system, the envelope B of the input currentwaveform of the inverter varies exponentially. Average current flowingin this stage is shown by W and a current having 6 times frequencycomponent as that of the inverter output frequency flows in the circuitin response with the variation of this envelope B.

FIG. 6 shows input current waveforms of an inverter made in accordancewith the present invention. FIG. 6(a) and FIG. 6(b) show respectivelythat for a case in which an exponential function waveform A₆ shown inFIG. 4 is applied.

In an embodiment in which an exponential function waveform A₆₁ shown inFIG. 6(a) is applied, the level signal frequency is 6 times of theinverter output frequency (m=3, 2m=6) and the frequency of the carriersignal A₃ is further integer multiple thereof, i.e. in the illustratedexample 48 times multiple. The pulse width T is determined depending onthe crossing points of the carrier signal A₃ and the exponentialfunction waveform A₆₁. The pulse width decreases exponentially as shownin the drawings and the average value W₁ of the inverter input currentfor each one of the pulses can be made substantially constant as shownin FIG. 6(c). Accordingly, the pulsating current component having 6times frequency with the inverter output frequency is suppressed and atthe same time the commutating peak current is also decreased by thedecrease of the pulse width as will be explained later on.

There still remains the chopping frequency components. However, by thefiltering effect for the higher harmonics such components maysufficiently be suppressed so that a smooth dc current may flow at thesource side from the feeder line at the input side of the filter towardsthe return circuit. In this connection the dc level V₀ is so controlledthat the normal inverter output voltage is in proportion to the inverteroutput frequency just as same as the conventional modulation system. Thedepth of level ΔV of the exponential function waveform A₆₁ and its timeconstant T_(EX) may also be made to have corelation with the inverteroutput frequency like the dc level V₀. However, by adjusting thefundamental frequency component of the inverter input current to beminimum at an input frequency of the inverter coinciding operatingfrequency of the aforementioned security signal apparatus, and bymaintaining the depth of the level signal then and the time constantthereof, the current having the frequency component in problem cansubstantially be removed so that the object may be reached. In practice,the device may be designed to effect normal modulation from the time ofstarting when the filtering effect becomes inferior up to a time whenthe inverter input frequency passes the security signal frequency bandusing the commercial main frequency or a predetermined certainfrequency. By this the lower harmonic components can be decreased toreduce disturbing current component to the security signal devices. Theoverall device including inverter 7 can be miniaturized and light weightdevice may be realized without the need of enlarging the filtercapacity. Accordingly, the device of the present invention isparticularly suitable for energizing an inverter of an electric train.

An exponential function waveform A₆₂ shown in FIG. 6(b) is analternative of the case of FIG. 6(a) and by using this waveform A₆₂ anidentical modulation result can be obtained. Namely, the exponentialfunction waveform A₆₂ and the carrier signal A₃ are both reversed thepolarity, the pulse width is determined in a range that the carriersignal A₃ is lower than the level signal A₆₂ of the exponential waveformand the pulse width T shown in FIG. 6(c) can be obtained. In practice inthe circuit arrangement shown in FIG. 3, the output signal of thetriangle wave generator 4, the dc level V₀ of the adder 11 and theoutput of the function generator 10 may be reversed.

FIG. 7 shows modified embodiment of the modulation of the presentinvention. In this embodiment, in place of the exponential functionwaveforms A₆₁ and A₆₂ shown in FIGS. 6(a) and 6(b), respectively, thelevel signal A'₆ is given in sawtooth waveform. In this case themodulation level signal A'₆ is given in sawtooth form as shown in thedrawing and the pulse width T of the inverter input current of each ofthe pulses in one cycle varies linearly along an electric angle θ.According to this embodiment, the average value W₂ is not a constant oneas can be seen from FIG. 7(b). But it affords a sufficient improvementover the conventional waveform as shown in FIG. 5 so that the object ofthe invention can be achieved in practice.

For further improving the effect of the modulation system of the presentinvention it is desirable to realize a generator being able to alwaysderive an analogous waveform irrespective to the variation of theinverter output frequency instead of the function waveform generator 10shown in FIG. 3. FIG. 8 is an embodiment for realizing such a system.

In FIG. 8, reference numeral 12 designates a function waveformgenerator, which comprises an address instructor 12a, a waveform memoryelement 12b and a digital-to-analog (DA) converter 12c. The memoryelement 12b memorizes the exponential function waveform as shown in thedrawing and the digital signal read out from the waveform memory element12b is converted into an analog signal by the DA converter 12c. As sameas the device shown in FIG. 3, this analog signal output is added withthe level signal A₂ by the adder 11. The address instructor 12asuccessively renews the output signal by a pulse signal having frequencysubstantially higher than the inverter output frequency, for instance,by an input of the frequency divider 2. By this the content of thewaveform memory element 12b is read out. Further in order to make theoutput of the DA converter to be desired repetition frequency, thereturning to the initial condition at the desired time, for instance atthe reversing time of the output of the final stage is also effected bysaid pulse signal.

As has been explained in the foregoing, the present invention is torealize a novel modulation system of a PWM inverter device in which thecurrent commutating duty of the inverter can be substantially reduced soas to decrease current in lower order harmonic components and being ableto cover the decrease of filtering effect at low frequency time. Theinvention thus is able to realize a small and light weight device.

What is claimed is:
 1. A PWM inverter device for energizing ac motorwith variable voltage and variable frequency output obtained from a dccurrent source by pulse width adjustment, characterized in that thedevice comprises an oscllator for generating a series of output pulsessynchronized with output frequency of the inverter and having frequencyintegral multiple of the output frequency, a function waveform generatorfor generating level signal having frequency of 2 m times (m being aninteger representing the number of phases of the inverter device) of theinverter output frequency in response to the output of said oscillator,a triangle waveform oscillator for generating triangle wave shapedcarrier signal having frequency of integral multiple of the levelsignal, a modulator for comparing and mixing said level signal with saidcarrier signal, wherein said level signal is generated in a functionalform in order that the pulse width of the inverter output voltagedecreases monotonically and to form repeated waveforms by a choppinginstruction of the modulator output.
 2. A PWM inverter device as claimedin claim 1, wherein said level signal generated by said functionwaveform generator and synchronized with said inverter output frequencyand having 2 m times frequency with that of the output frequency isgenerated as a sawtooth waveform.
 3. A PWM inverter device as claimed inclaim 1, wherein said function waveform generator comprises a memoryelement for memorizing said function waveform and the signal isgenerated by reading out the memorized content.
 4. A PWM inverter deviceas claimed in claim 1, wherein said level signal generated by saidfunction waveform generator and synchronized with said inverter outputfrequency and having 2 m times frequency with that of the outputfrequency is generated as an exponential waveform.
 5. A PWM inverterdevice for energizing ac motor with variable voltage and variablefrequency output obtained from a dc current source by pulse widthadjustment, characterized in that the device comprises an oscillator forgenerating a series of output pulses synchronized with output frequencyof the inverter and having frequency integral multiple of the outputfrequency, a function waveform generator for generating level signalhaving frequency of 2 m times (m being an integer represening the numberof phases of said inverter device) of the inverter output frequency inresponse to the output of said oscillator, a triangle waveformoscillator for generating triangle wave shaped carrier signal havingfrequency of integral multiple of the level signal, a modulator forcomparing and mixing said level signal with said carrier signal, whereinduring a period from starting time to a time when the inverter inputfrequency passes a certain predetermined frequency said level signal isgenerated in a function waveform in order that the pulse width of theinverter output voltage decreases monotonically and to form repeatingwaveforms by the chopping instruction.